Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications and documents is incorporated by reference herein.
Telomeres, the protein-DNA structures physically located on the ends of the eukaryotic organisms, are required for chromosome stability and are involved in chromosomal organization within the nucleus (See e.g., Zakian, Science 270:1601 [1995]; Blackburn and Gall, J. Mol. Biol., 120:33 [1978]; Oka et al., Gene 10:301 [1980]; and Klobutcher et al., Proc. Natl. Acad. Sci., 78:3015 [1981]). Telomeres are believed to be essential in such organisms as yeast and probably most other eukaryotes, as they allow cells to distinguish intact from broken chromosomes, protect chromosomes from degradation, and act as substrates for novel replication mechanisms. Telomeres are generally replicated in a complex, cell cycle and developmentally regulated manner by telomerase, a telomere-specific DNA polymerase. Telomerase-independent means for telomere maintenance have, however, been described. In that telomere loss is associated with chromosomal changes such as those that occur in cancer and aging, the study of telomeres and mechanisms contributing to their regulation have been at the center of intense investigations.
Telomeric DNA: In most organisms, telomeric DNA has been reported to consist of a tandem array of very simple sequences, which in many cases are short and precise. Typically, telomeres consist of simple repetitive sequences rich in G residues in the strand that runs 5′ to 3′ toward the chromosomal end. Heterogenous telomeric sequences have, however, been reported in some organisms. In addition, the repeated telomeric sequence in some organisms is much longer. The telomeric DNA sequences of many organisms have been determined (See e.g., Zakian, Science 270:1601 [1995]) and such studies have revealed that only limited consensus exists among these sequences (Zakian, supra). The average amount of telomeric DNA also varies among organisms. Moreover, in most organisms, the amount of telomeric DNA fluctuates. Heterogeneity and spontaneous changes in telomere length are thought to reflect a complex balance between the processes involved in degradation and lengthening of telomeric tracts. In addition, other factors including genetic and nutritional influences may cause increases or decreases in telomeric length (Lustig and Petes, Natl. Acad. Sci., 83:1398 [1986]; and Sandell et al., Cell 91:12061 [1994]). The inherent heterogeneity of virtually all telomeric DNAs suggests that telomeres are not maintained via conventional replicative processes.
Telomere Replication: Complete replication of the ends of linear eukaryotic chromosomes presents special problems for conventional methods of DNA replication. For example, conventional DNA polymerases cannot begin DNA synthesis de novo, rather, they require RNA primers which are later removed during replication. In the case of telomeres, removal of the RNA primer from the lagging-strand end would necessarily leave a 5′-terminal gap, resulting in the loss of sequence if the parental telomere was blunt-ended (Watson, Nature New Biol., 239:197 [1972]; Olovnikov, J. Theor. Biol., 41:181 [1973]). However, the described telomeres have 3′ overhangs (Klobutcher et al., Proc. Natl. Acad. Sci., 58:3015 [1981]; Henderson and Blackburn, Mol. Cell. Biol., 9:345 [1989]; and Wellinger et al., Cell 72:51 [1993]). For these molecules, it is possible that removal of the lagging-strand 5′-terminal RNA primer could regenerate the 3′ overhang without loss of sequence on this side of the molecule. Loss of sequence information on the leading-strand end would, however, occur due to the lack of a complementary strand to act as template in the synthesis of a 3′ overhang (Zahler and Prescott, Nucleic Acids Res., 16:6953 [1988]; Lingner et al., Science 269:1533 [1995]).
While conventional DNA polymerases cannot accurately reproduce chromosomal DNA ends, specialized factors exist to ensure their complete replication. Telomerase (TERT) is a key component in this process. Telomerase is a ribonucleoprotein (RNP) particle and polymerase that uses a portion of its internal RNA moiety as a template for telomere repeat DNA synthesis (Yu et al., Nature 344:126 [1990]; Singer and Gottschling, Science 266:404 [1994]; Autexier and Greider, Genes Develop., 8:563 [1994]; Gilley et al., Genes Develop., 9:2214 [1995]; McEachern and Blackburn, Nature 367:403 [1995]; Blackburn, Ann. Rev. Biochem., 61:113 [1992]; Greider, Ann. Rev. Biochem., 65:337 [1996]). The activity of this enzyme depends upon both its RNA and protein components to circumvent the problems presented by end replication by using RNA (i.e., as opposed to DNA) to template the synthesis of telomeric DNA. Telomerases extend the G strand of telomeric DNA. A combination of factors, including telomerase processivity, frequency of action at individual telomeres, and the rate of degradation of telomeric DNA, contribute to the size of the telomeres (i.e., whether they are lengthened, shortened, or maintained at a certain size).
Notably, telomere replication is regulated both by developmental and cell cycle factors. It has been hypothesized that aspects of telomere replication may act as signals in the cell cycle. For example, certain DNA structures or DNA-protein complex formations may act as a checkpoint to indicate that chromosomal replication has been completed (See e.g., Wellinger et al., Mol. Cell. Biol. 13:4057 [1993]). In addition, it has been observed that in humans, telomerase activity is not detectable in most somatic tissues, although it is detected in many tumors (Wellinger, supra). Thus, telomere length may serve as a mitotic clock, which serves to limit the replication potential of cells in vivo and/or in vitro. In light of the contribution of telomerase to maintenance of telomere function and activity, telomerase is a bona fide target for development of agents directed to cancer therapy and/or slowing of aging processes.